I once again find myself in the debt of someone that had posted on the forum about the recommendation to use Fast Orange to clean residue.
I just did about 50 key tags for a customer and of course the leather had residue. I used the Fast Orange towlets to clean the leather and was amazed at how easy it works. They are also good at cleaning up the non-optical parts of the interior of the .
The folks on the forums here are AMAZING!